Armstrong Renata, 23, was sentenced to seven years jail and will be eligible for parole in about August 2021.

Steven Miller, the father of 18-year-old Cole Miller, with Cole’s uncle Michael and aunt Allison Miller, speaks to the media in the days shortly after the tragic death of his son.

"We watched justice serve its course today," Mr Miller said.

Asked what he thought about Renata reading an apology letter in court, Mr Miller said: "I think if I was him, I would've apologised too."

He said the sentence handed down on Friday was "prepared professionally".

"Look, the sentence is the sentence," Mr Miller said

He said his heart went out to all victims of violence.

Mr Miller said Australia as a society had a serious problem with violence which the country needed to address.

EARLIER: AS he sentenced a one-punch killer to seven years behind bars, a justice has said Cole Miller's family face a life sentence themselves.

Justice Helen Bowskill told Armstrong Renata he will have to serve 80% of that sentence.

"One moment has stolen the life of a young man and destroyed the life of his family and close friends" she said.

The 23 year-old fatally coward-punched Mr Miller, a Sunshine Coast 18 year-old, in January last year.

"Your conduct on that night was cowardly, gratuitous and unprovoked," the judge told Renata at his sentencing on Friday.

Renata and others "who fail to control their violent actions" must be punished, Justice Bowskill said,

In June, Renata pleaded guilty to unlawful striking causing death.

"There is no escaping that this is an incredibly serious offence," the judge said.

Renata faced the possibility of life imprisonment, or might have had to serve 80% or 15 years of any jail sentence.

Justice Bowskill said the fatal punch was a tragedy for Mr Miller and his family, and also a tragedy for Renata, whom she described as a young man who made a "terrible" decision.

Renata's guilty plea, remorse, and the fact he spend six months in maximum security custody counted as mitigating factors.

Because Renata has been in custody for some 21 months, he will be eligible for parole in August 2021.

EARLIER: COLE Miller's mum Mary-Leigh says her "heart shattered into a million pieces" when her teenage son died in a one-punch attack.

"My life as I knew it came to an end," she said in a victim impact statement read at the sentence of her son's killer.

"I endure constant pressure from the world around me to move on.

"I cannot help but blame myself…. I could never have imagined this would become the new normal."

Mrs Miller said she had "continual and desperate yearning" to return to the way life was before Armstrong Renata delivered a fatal blow during a night out in Brisbane.

Cole's father Steven told Brisbane Supreme Court Cole's life and death "has touched so many people".

"This a statement that no parent ever wants to provide," he said.

Renata put his head in his hands as Mr Miller began reading.

A portrait of 18 year-old one-punch victim Cole Miller is placed near the coffin during his funeral service last year. Cole died in hospital on January 4 from massive brain trauma a day after allegedly being hit in Brisbane's Fortitude Valley. (AAP Image/Dan Peled)

"My greatest concern … is that my three children won't be able to lead normal lives," Mr Miller told the court

"I pray that my children can move on and survive."

Mr Miller said Cole's friends and family still felt the repercussions of Cole's shocking, unnecessary death.

"Their shock, their disbelief, and their sadness was so hard to watch. It has been a long 20 months since then."

"I must keep going forward and lead the way for my children," he added on Friday morning.

Cole's friend Nick Pace said: "I'm still being kept up at night thinking about the incident."

Renata's barrister Angus Edward said: "Nothing I submit now can take away from the tragedy and loss."

He said Renata had shown "genuine remorse" and entered an early guilty plea.

"He was intending to go and hand himself in to police."

He said Renata had a relatively minor criminal history.

Mr Edwards said Renata spent six months in high security custody because he was wrongly accused of punching jail guards.

"That wasn't true. My client didn't do it … but as a result of that wrong charge, he ended up in the maximum security unit."

Renata, born in New Zealand, moved to Australia in 2008.

He had a son aged five who lived with his former partner.

'I can't imagine the pain and trauma I've caused'

THE young man who killed Cole Miller says he doesn't believe he will ever forgive himself.

Armstrong Renata, 23, read a letter of apology as he was being sentenced for unlawful striking causing the death of the Sunshine Coast teenager